Mitochondrial DNA is inherited separately from nuclear DNA (the chromosomes. ) Fragments of mitochondrial DNA are known to get inserted into nuclear DNA to form NUMTs, i.e. nuclear pseudogenes of the mtDNA. The insertion of a NUMT is a rare event.

Humans, gorillas and chimpanzees share a NUMT on chromosome 5, dubbed “ps5” by Konstantin Popadin et al (probably meaning “pseudogene on chromosome 5”), and a comparison of the NUMT with the mtDNAs of the great apes implies that, around the time of divergence between humans and chimpanzees, there was interbreeding of individuals whose mtDNA had diverged as much as ~4.5Myr prior to the interbreeding event, which matches the estimate for when the human-chimpanzee lineage diverged from gorillas.

The scenario that fits best with the available data (including genetic data and the fossil record) is one where mtDNA was transferred, in a hybridization event, from gorillae to the human-chimpanzee lineage at around 6Ma, where the ps5 NUMT was created in the acceptor species from its ‘old’ mtDNA, which later gets replaced from the population by the ‘new’ mtDNA of the introgressing species, gorillae. The ancient mtDNA of the human-chimpanzee lineage would no longer be present in either gorilla, human or chimpanzee, and the NUMT that was created from it would be present in all three lineages.

References

Mitochondrial pseudogenes suggest repeated inter-species hybridization among direct human ancestors (2017)